The wife has a Mercedes c320. The add oil light came on. I call the dealer that serviced it last and they said Mobil 1 synthetic 5w40. I go to O'reillys and they have mobil 1 syn 5w40 that is specified for turbodiesel motors. They didn't have any other 5w40s in the Mobil1 brand. The question is will the diesel stuff be the same? I figure 5w40 is 5w40 right? I don't want to put the wrong stuff in and the wife leaves early before the dealership opens tomorrow.

You should be fine blending any synthetic oil with your mobil 1. The diesel stuff is actually a different oil in alot of cases. Diesels require a different additive package. I would have to look into the specifics with Mobil 1. Sometimes they just repackage. We transitioned from Mobil 1 here at the shop to Total synthetics. Their stuff covered more of the different manufacture requirements and tested very well. Stick to the right weight and pick up a good synthetic. You wont have issues. Castrol Syntec would be fine as well as a bunch of others. We get customers that run whatever's the cheapest in their cars and I've yet to see an issue. The only problems I've see have been from people running 4-5 qts low or from 30k oil changes. Don't laugh.... it happens all the time. I can't even tell you how many cars come in with only 1-2 qts in them.

OK, I think I'll take this stuff back and get the right stuff at a different shop. My old Ford always ran at about 4 quarts out of 6, I think every time I started it half a quart burned up past the rings out of the exhaust. Unfortunately I think that is starting to happen in the wife's car. It's always a little smoky in the morning fire ups.

You could always go to a heavier weight oil since it's not a cold climate car. We just changed my brothers S4 over to 10-60 to help with oil consumption, and it worked great. I don't remember what Mobil 1 offers, but you could try 15-50.